Post subject: Transmission Line Question Posted: Fri Mar
07, 2008 2:46 pm
Joined: Fri Mar 07, 2008
In his book, Prof. Guillermo Gonzalez writes:
V(x)=V+(x) + V-(x) and
I(x)=I+(x) - I-(x)
is there a minus sign in the second equation? The equations say the
voltage at a point on the transmission line is the sum of the forward
wave voltage V+(x) and the reflected wave voltage V-(x). But the current
is the difference of the current of the forward wave I+(x) and the current
of the reflected wave I-(x). It seems to me the voltage and the current
are vector quantities, i.e. they have a phase. So why the minus sign?
Why not just sum them?
Post subject: Posted: Fri Mar 07, 2008 4:30 pm
Joined: Wed Feb 22, 2006 3:51 pm
If I understand
it correctly, it's to make the direction of the power flow work out
correctly. Power doesn't really have a phase, but power flow definitely
has a direction, and I think the signs are set up to give that.
Hope this helps!
subject: Posted: Fri Mar 07, 2008 5:18 pm
Joined: Fri Mar 07, 2008 2:29 pm
correct. The apparent current has to be defined as the incident wave
current minus the reflected wave current to get the power relationships